What is witnessed today is an increased demand for helium cryogenic systems displaying a higher than ever before performance and good economy in terms of capacity, power requirements, operational reliability and so on. This fact dominating the cryogenic scene owes its origin mainly to the rapid pace of research and development work in the field of applied cryogenics associated with the use of the phenomenon of superconductivity in creating electrical machinery, powerful magnets, power transmission lines, electronic devices and also with the use of liquid hydrogen on a large scale.
Since superconductive devices require temperatures as low as 1.5.degree. to 15.degree. K. for their operation and taking into account that the power requirements of cryogenic systems used to cope with great refrigerative loads amount to hundreds and thousands of kilowatts, any effort to reduce the power requirements of cryogenic systems and to modify these systems so as to improve their reliability, reduce their weight, overall dimensions, etc, is appreciated. In some cases it appears to be essential to lower the temperature level achievable in the system under the conditions of good economy.
The power requirements of a low-temperature system are commonly given as the ratio of energy consumed, mainly for driving the compressor, per unit of energy removed by the refrigerator. Both these values are commonly measured in watts, and the ratio referred to is termed as actual power requirements expressed by a dimensionless figure (W/W).
For those who are versed in the art it is known that the energy removed by the refrigerator is expressed in terms of refrigerating capacity which is the amount of heat removed by the refrigerator per unit time at the specified temperature level.
There is known a method of producing supercold temperature comprising a number of operations which will be now considered by way of example in describing a helium cryogenic system capable of producing a temperature equal to the boiling point of liquid helium which is 4.2.degree.-4.5.degree. K.
Gaseous helium is compessed to between 20 and 30 bars in a compressor, and the compressed helium constituting the incoming stream heading for a refrigerative load is cooled down to around 100.degree. K. by a low-pressure return stream heading in the reverse direction, i.e., away from the refrigerative load. Next, the incoming stream is split into two streams termed the main stream and subsidiary stream. The subsidiary stream is expanded in expanders with the removal of energy and used to compensate for irreversible losses and to cool down the main stream step by step, the number of cooling stages being determined by the number of the expanders employed in expanding the subsidiary stream. Expanders are sometimes replaced by a bath with a liquid refrigerant, for example, nitrogen or any other substance whose boiling point is sufficiently low to enable the process of cooling to take place. The main stream passes through all the cooling stages and then reaches the liquefaction stage where it is additionally cooled and expanded with liquefaction.
The liquid helium is used to sustain the refrigerative load and, on removing heat, evaporizes. The vapour constitutes the return stream which flows at a temperature of 4.3.degree.-4.5.degree. K. in a countercurrent manner relative to the main and subsidiary streams, absorbs heat in the heat exchangers of all the stages, merges while underway with the subsidiary stream expanded in the expanders and enters the compressor at a temperature around 300.degree. K. and under atmospheric pressure to be compressed therein. This completes the cycle which is then repeated.
In one version of the method described, the expansion of the main stream accompanied by its liquefaction in the liquefaction stage takes place due to throttling while in another version this is achieved by expansion with the removal of energy. The throttling of liquid is a process used for years, and the process of expansion into the region of saturation in an engine doing external work is referred to in "Cryogenic Engineering" by R. B. Scott, D. Van Nostrand Co. Inc., Princeton, 1959, where the author, describing by way of example a helium liquefaction system, points out all the advantages offered by the expansion process when this process is compared with the throttling process.
In the described process of producing supercold temperature, the energy consumed in compressing the gas is partly recovered in the refrigerator and is partly used to compensate for various losses, i.e., is lost with no useful purpose and contributes to gaining entropy by the helium. Termed as the losses due to the irreversibility of the process, these losses result from heat transfer at temperature gradients other than zero, from friction in the stream of helium and from other causes.
Thermodynamically recoverable in cryogenic systems are less than 20% of the energy consumed, and balance being lost in compensating for irreversible losses. The bulk of these losses are losses due to temperature difference, particularly in the region of supercold temperatures. Computations indicate that the losses incurred in the liquefaction stage are roughly at balance with the net effect, i.e., with the refrigerating capacity, and that a reduction in these losses is conducive to an increase either in the effectiveness of the method of producing supercold temperature or in the performance of the system realizing this method. The losses of energy due to the irreversibility, for example, of the process of heat exchange are betrayed by the inability of the return stream to cool down the incoming stream sufficiently low as this may be the case taking into account the temperature of the return stream. A certain amount of energy is lost due to incomplete heat transfer so that an increase of the entropy of the helium is incurred and, as a result, the incoming stream is transformed into liquid fluid fed to sustain the refrigerative load in a quantity which is smaller than this can be anticipated. This implies that the refrigerating capacity of the system is an inadequate one. The quantity of liquid fluid can be increased under the conditions of loss of energy due to the irreversibility if more energy is consumed, as this may be the case when a greater amount of gas is being compressed in the compressor. The losses due to the irreversibility of the process of heat transfer increase with the increase in the ratio of the temperature difference to the absolute temperature.
In realizing the known method of producing supercold temperature wherein preference is given to throttling as a means of expanding and liquefying, the optimum pressure to be maintained in the compressor is around 25 bars which provides for the highest refrigerating capacity of the system or for the lowest actual power requirements in producing supercold temperature. Yet, the losses due to the irreversibility of the process of heat transfer appear to be rather high in this case, being brought about in the liquefaction stage by a great difference between the temperature of the incoming stream and that of the return stream. Moreover, a loss of energy due to the irreversibility is unavoidable in the course of intermediate throttling aimed at minimizing this difference in the temperatures, the energy of the compressed gas consumed during this intermediate throttling, which reduces the pressure from 25 bars to between 3 and 5 bars, mainly for overcoming the friction in the means of throttling the Joule-Thompson valve. The main cause of all these losses is a substantial difference between the heat capacity of the incoming stream and that of the return stream in the liquefaction stage.
Even when the main stream is expanded to liquefaction with the removal of energy the losses incurred due to the irreversibility of the process of heat transfer remain to be of significant nature. As indicated in the abovementioned book by R. B. Scott, this drawback stems from the fact that even in an idealized example there does always exist a substantial between the temperatures of the incoming and return streams in the liquefaction stage, this difference being especially pronounced at the lowermost temperatures of the streams and incurring a gain in entropy by a considerable amount. This temperature difference is not influenced by the efficiency of the heat exchanger and will be present in the most ideal case, i.e., in one when the difference between the temperatures of the streams at the other end of the heat exchanger is equal to zero. When the working fluid is helium, the difference between the temperatures of the incoming or, as in the case under consideration, of the main stream fed under a pressure of 25 bars and the return system admitted under a pressure of 1.3 bar is around 1.5.degree. K. at one end of the heat exchanger whereat the return stream is entering at a temperature of 4.5.degree. K. This temperature difference increases to 2.5.degree. K. toward the middle of the heat exchanger and falls to less than 0.5.degree. K. at the opposite end of the heat exchanger. Said temperature difference of 0.5.degree. K. is to be regarded as the maximum allowable value consistent with good effectiveness of the method of producing supercold temperature.
The disadvantage referred to above has a direct bearing on the fact that the low temperature of the return stream is utilized irrationally over the range between the boiling point, which is 4.5.degree. K. for helium, and the temperature of the compressed stream before its expansion with liquefaction (around 6.degree. K. for helium). This leads to a decrease in refrigerating capacity or to an increase in power requirements. A significant difference between the heat capacity of compressed main stream and that of expanded return stream aggravates the situation, said difference decreasing with a decrease in the pressure of the compressed incoming stream. In the known method, any reduction of the pressure of the fluid downstream of the compressor below 20 to 25 bars is impractical because this will impair the performance, resulting in higher actual power requirements and lower refrigerating capacity, and will lead to a substantial increase in overall dimensions of both the compressor and the system realizing the method. It is sufficient to mention that, for example, a decrease in the pressure of the fluid from 20 to 8 bars results in a refrigerating capacity which is only 1/2 of what is commonly obtained whereas the surface of the heat exchanger increases two-fold and the amount of the energy consumed goes up by 30%.
It stands thus to reason that, in realizing the known method, a specified refrigerating capacity is obtainable only by increasing the amount of the gas compressed or, in other words, by increasing the amount of the energy consumed. On the other hand, if the amount of the gas compressed is specified, i.e., using the compressor given, the realization of the known method provides for no other alternative than too low refrigerating capacity and high actual power requirements.